It is known to make foamed materials from synthetic plastics such as polyurethane. Basically, there are two processes available. In the first process, the starting composition comprising monomers and foaming agent is polymerized and simultaneously foamed in a single-stage process. According to this process useful products can be obtained only if the monomers polymerize rapidly and thus form a mechanically stable product which may be foamed by the gases or vapours of the foaming agent, whereby stable cells are formed.
In the second known process the monomeric starting material is initially polymerized with the admixed foaming agent without being foamed, whereupon in a second stage the polymer is brought into a plastic or fluid condition by heating while simultaneously the foaming agent is either evaporated or decomposed, so that gas bubbles or cells are formed in the synthetic material and a foamed plastic is obtained.
In the case of polymethacrylates none of these known processes leads to the desired success. In the first-mentioned process, it is impossible to keep the bubbles (cells) in a stable state during the polymerization of the acrylic foam. Due to the polymerization the surface tension of the bubbles increases so that they burst and combine with each other, thus causing the foam to collapse before it is cured.
The known two-stage process also does not lead to the desired success in the case of polymethacrylates. According to the British Patent Specification No. 1,448,621 and the French Patent Specification No. 1,423,844 a polymethacrylate is to be foamed by initially polymerizing a methacrylate with a foaming agent admixed thereto under conditions which inhibit the activation of the foaming agent. In a second stage the cured polymethacrylate is heated to the softening temperature at which the foaming agent will be activated. But because of the high internal pressure the foamed plastic substantially comprises only small bubbles or cells. Experiments have shown that during foaming the polymethacrylate composition is blown to all sides to result in a foam with a wavy and warped surface. The products obtained therefrom furthermore have little transparency and permit the light to pass therethrough only diffusely because it is reflected by the numerous small bubbles or cells.
There is a demand for a clear, transparent foamed plastic on a polymethyl methacrylate basis, which has a high transparency and at the same time good mechanical stability and high thermal insulation property as well as a smooth surface structure and which may be obtained in the form of sheets, boards or slabs, shaped articles and laminated or multilayer articles having a foamed core. It is impossible with any of the known processes to make such a foamed plastic, which is also fundamentally different with respect to its structure from known transparent extruded laminated boards of synthetic materials, which comprise in their interior either webs or closely adjoining very small or capillary-type tubes.